Internal combustion engines are generally equipped with at least one oxygen sensor which may be arranged at the inlet side in the intake system and/or at the outlet side in the exhaust-gas discharge system. The knowledge of the oxygen concentration O2 serves different purposes within the context of the control of internal combustion engines.
A sensor arranged in the intake system of the internal combustion engine may for example be used to control or regulate an exhaust-gas recirculation system, that is to say to adjust the recirculation rate.
Exhaust-gas recirculation (EGR), that is to say the recirculation of combustion gases from the outlet side to the inlet side of the internal combustion engine, is used in particular to reduce nitrogen oxide emissions. With increasing recirculation rate, the nitrogen oxide emissions can be considerably reduced.
Here, the exhaust-gas recirculation rate xEGR is determined as xEGR=mEGR/(mEGR+mfresh air), where mEGR denotes the mass of recirculated exhaust gas and mfresh air denotes the supplied fresh air. To obtain a considerable reduction in nitrogen oxide emissions, high exhaust-gas recirculation rates xEGR are required which may be of the order of magnitude of up to 70%.
In order for the sensor to be impinged on by charge air, which may contain recirculated exhaust gas in addition to the fresh air, and not exclusively by fresh air, the sensor may be arranged downstream of the opening of the recirculation line of the EGR into the intake line.
The sensor detects the oxygen concentration O2 in the charge-air flow, which concentration can be taken into consideration in an equation for determining the proportion Fintake of the charge air fraction resulting from the combustion, and/or for determining the recirculation rate xEGR. Using the sensor, it is therefore possible for the recirculation rate xEGR of the EGR to be adjusted, that is to say for a shut-off element, which is preferably arranged in the recirculation line and which serves as an EGR valve for adjusting the recirculation rate, to be actuated.
Under certain assumptions, it is possible in this way to realize closed-loop control of the EGR, for example if only a single exhaust-gas recirculation line is provided. Furthermore, the oxygen concentration O2 detected by means of a sensor or the proportion Fintake may be used to determine the nitrogen oxide concentration CNOx,exhaust in the exhaust gas, that is to say the untreated emissions of nitrogen oxides NOx. Here, it is possible to dispense with an expensive NOx sensor, which is arranged in the exhaust-gas discharge system, for determining the nitrogen oxide concentration CNOx,exhaust in the exhaust gas and/or for regulating the EGR valve, that is to say for regulating the exhaust-gas quantity recirculated.
In the case of an internal combustion engine which is supercharged by an exhaust-gas turbocharger and which is equipped with a low-pressure EGR arrangement in which a recirculation line branches off from the exhaust line downstream of the turbine and opens into the intake line upstream of the compressor, the oxygen sensor is preferably arranged downstream of the compressor and upstream of a charge-air cooler which may be provided.
On the one hand, the recirculation of aftertreated exhaust gas by low-pressure EGR ensures that the sensor is not contaminated with soot particles contained in the exhaust gas and/or by oil contained in the exhaust gas. On the other hand, the charge air is at an elevated temperature after the compression in the compressor. The latter fact helps the sensor, which is generally equipped with an electric heating mechanism, to reach and maintain its operating temperature.
Aside from the recirculation of hot exhaust gases, however, further measures are necessary in order to reduce the pollutant emissions. Internal combustion engines are often therefore equipped with a plurality of exhaust-gas aftertreatment systems.
For the oxidation of the unburned hydrocarbons (HC) and of carbon monoxide (CO), oxidation catalytic converters are provided in the exhaust system. If nitrogen oxides (NOx) are additionally to be reduced, this may be achieved, in the case of Otto engines, through the use of a three-way catalytic converter, which however for this purpose requires stoichiometric operation (λ≈1) within narrow limits.
For the reduction of the nitrogen oxides, either selective catalytic converters are used in which reducing agent is introduced into the exhaust gas in a targeted manner in order to selectively reduce the nitrogen oxides, or nitrogen oxide storage catalytic converters are used in which the nitrogen oxides are initially absorbed, that is to say collected and stored, in the catalytic converter before then being reduced during a regeneration phase with a lack of oxygen.
According to the previous systems, to minimize the emissions of soot particles, use is made of so-called regenerative particle filters which filter the soot particles out of the exhaust gas and store them, with said soot particles being burned off intermittently during the course of the regeneration of the filter.
To monitor the abovementioned exhaust-gas aftertreatment systems and/or to check their functional capability, an oxygen sensor arranged at the outlet side in the exhaust-gas discharge system of the internal combustion engine may be used or may be expedient, because reactions taking place in the aftertreatment systems change the oxygen concentration O2 in the exhaust gas, that is to say in the gas flow relevant in the present case.
To improve the emissions behavior of an internal combustion engine and to basically improve the operation of the internal combustion engine, high-quality control and/or regulation may be provided, that is to say to control and/or regulate as exactly as possible the exhaust-gas aftertreatment systems provided in the exhaust-gas discharge system and an exhaust-gas recirculation system which influences the composition of the charge air.
High-quality control and/or regulation assumes that the signals output by the sensors are as accurate as possible, in particular, the oxygen concentration detected by the oxygen sensor and/or the oxygen concentration provided to the engine controller for further use has the least possible error.
However, conventional oxygen sensors are calibrated to output an oxygen concentration assuming a single, constant pressure of oxygen within the sensor. Frequently, the pressure of the oxygen in the sensor fluctuates, resulting in errors in the measured oxygen concentration.
The inventors have recognized the issues with the above approach and offer a method to at least partly address them. In one embodiment, a method for determining oxygen concentration in a gas flow of an internal combustion engine which is equipped with an engine controller and an oxygen sensor comprises determining the oxygen concentration of the gas flow in a ceramic measurement cell of the sensor by current which can be detected by measurement and which flows when a constant voltage is applied and maintained, and correcting the oxygen concentration based on a pressure at the measurement cell.
In this way, the method according to the disclosure takes into consideration that the pressure pSens in the gas flow and therefore in the measurement cell of the sensor varies and has an influence on the oxygen concentration O2,sens provided by the sensor. This permits considerably improved control and/or regulation of the internal combustion engine.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.